Economy and quality are concerns in the art of paper making. Those skilled in the art are always seeking to optimize these two features of the paper making process. The basic paper making process is known to those skilled in the art. For the sake of completeness, a general description of the paper maker's art is presented herein.
The material that paper is made from is called "furnish". Furnish is mostly fiberous material, to which is sometimes added mineral fillers, and chemical additives. The most common fiberous material is wood pulp. Grasses, cotton, and synthetics are used occasionally. Wood is made up of fibers (cells) which are held together with lignin.
Wood pulp is made by either chemically or mechanically separating the fibers. Different methods give variations in quality. Chemical wood pulp is typically of high quality. It as long smooth fibers, but is expensive to produce. Mechanical pulp is less expensive. The fibers are shorter, often with a very rough surface. Recycled pulp is made by slurrying waste paper in water. The fibers come out shorter and more degraded than they were originally. A variety of methods are used to bleach the fibers whiter, and remove contaminants. Some of these methods further degrade the fibers. Extremely short fibers are called "fines" and are less than 1/100 of an inch long. Fines can amount to over 50% of the total fiber.
The wood pulp or furnish is transferred to the paper machine as a slurry of about 4% fiber and 96% water and is called "thick stock". Mineral fillers may be added to this slurry. A typical addition is 10% filler, which is commonly either kaolin clay, or calcium carbonate (e.g., chalk). These fillers are very small particles, typically around 1 micron in size. Chemicals are then added to improve the properties of the paper, such as strength, water resistance or color.
At this point the furnish is ready to be added to the paper machine. In order to make paper, the furnish is further diluted down, to approximately 1.0% solids. This is referred to as "thin stock". The "thin stock" goes through screens and cleaners which impart a great deal of shear into the slurry. The "thin stock" then goes into the "headbox" which delivers the slurry onto a moving "forming" fabric or "wire".
After the furnish is put on the forming fabric or "wire", most of the water is removed by gravity and vacuum. The fines (much of the mechanical and recycled fiber) and all of the filler are small enough to go through the fabric or "wire". In order to keep these particles in the paper, they must be flocculated into larger particles.
While on the "wire" the solids content is raised up to around 15%. The paper is then run through presses that squeeze more water out to give solids of approximately 40-50%. The systems that use high molecular weight polymers give good dewatering on the wire, but often retard dewatering in the press section.
The final water removal stage uses steam dryers. A very small change in water removal in the press section makes a huge difference in the dryer section. The dryer section is the largest part of the machine, and typically limits the production rate.
Those skilled in the art of papermaking are always seeking ways to improve the paper manufacturing process. Specifically, U.S. Pat. No. 4,305,781, assigned to Allied Colloids Limited, discloses a method of making paper with improved drainage and retention properties of a cellulosic suspension. The method involves the addition of polymers having a molecular weight of above 500,000 to about 1,000,000 or above (column 3, lines 8-13) to the suspension. The polymers employed must be substantially non-ionic such as polyacrylamides (column 3, lines 14-16 and lines 27-33). The polymer is added the suspension after the last point of high shear prior to sheet formation (column 3, lines 66-68). The bentonite is added to the suspension in the thick stock, the hydropulper, or the re-circulating white-water (column 4, lines 3-8). The bentonite must be added prior to the polymer and at least one shear point will occur between the bentonite and polymer addition. The patent does not disclose the formation of small flocs.
U.S. Pat. No. 5,015,334, assigned to Laporte Industries Limited, discloses a colloidal composition and its use in the production of paper and paperboard (column 1, lines 9-11). The patent discloses that a polymer can be added to paperstock followed by adding bentonite to the paperstock without shearing between the addition of the polymer and the bentonite (column 2, lines 38-52 and column 4, lines 19-29). The polymer employed is a low molecular weight water-soluble, high charge density polymer having a molecular weight below 100,000 (column 3, lines 12-25).
Although the patent discloses that shearing is excluded between the addition of the polymer and bentonite in treating the paperstock, the patent does not disclose the formation of small flocs as the subject invention. Also, the patent employs low molecular weight polymers, not the medium molecular weight polymers, i.e., 100,000-2,000,000, as the process of the present invention.
U.S. Pat. No. 5,393,381, assigned to S N F, France, discloses a process for the manufacture of paper or cardboard having improved retention properties (column 1, lines 6-8). The process involves adding a branched, high molecular weight polymer such as a polyacrylamide (column 2, lines 43-56) to paper pulp followed by shearing the mixture (column 3, lines 28-34) then adding bentonite to the mixture (column 3, lines 34-37).
The high molecular weight branched polymers are employed because such polymers retain bentonite on a paper sheet better than non-branched polymers (column 2, lines 14-23).
The patent does not disclose employing the specific medium molecular weight branched polymers of the subject invention. Further, there is no discussion of the formation of small flocs. Additionally, the patent employs a shearing process between the addition of the polymer and the bentonite to the pulp unlike the present invention which eliminates the shearing process.
U.S. Pat. No. 5,676,796 assigned to Allied Colloids Limited, discloses a method for making paper or paperboard (column 1, lines 1-5). The method is directed to improving the retention, drainage, drying, and formation properties in paper making (column 3, lines 42-51). The process involves forming a thick cellulosic stock suspension and flocculating (column 3, lines 54-61 and column 4, lines 4-8) with a first polymer (column 6, lines 64-67 and column 7, lines 1-7). The first polymer employed can be a low anionic, a non-ionic, and a low and medium cationic polymer (column 9, lines 63-67 and column 10, lines 1-6). The thick stock is then diluted to form a thin stock (column 3, lines 62-63). The large flocs are then formed into small dense flocs in the thin stock by adding a coagulant such as a non-ionic polymer having a molecular weight of below 1,000,000 or 500,000 (column 4, lines 8-14, column 7, lines 8-33, and column 11, lines 42-51). In addition to the first and second polymer, bentonite can be added either before, with, or after the addition of the flocculant polymer (column 6, lines 50-63).
Preferably, the bentonite is added after the addition of the second polymer to the thin stock (column 4, lines 20-24). Prior to adding the bentonite, the stock is sheared (column 6, lines 58-63 and column 12, lines 36-39).
Although U.S. Pat. No. 5,676,796 discloses the formation of small flocs, by adding a polymer having a molecular weight of below 1,000,000, the method of the present invention employs a medium molecular weight polymer to form small flocs without the formation of large flocs by high molecular weight polymers as disclosed in U.S. Pat. No. 5,676,796. The present invention employs some high molecular weight polymers only to maintain the stability of the small flocs. Further, the method disclosed in U.S. Pat. No. 5,676,796 always employs shearing prior to adding bentonite. In contrast, the present invention does not employ shearing between adding the polymer and bentonite to the papermaking furnish.
Applicants' invention improves on the art because their program uses less polymer than a conventional bentonite program, improves press section dewatering, which increases the solids going into the dryers, and reduces drying requirements. Further, one less shear step is required.